1. Field of the Invention
This invention relates to the manufacture of flat glass. More particularly the invention relates to the manufacture of thin flat glass by the float process for example, float glass of thickness in the range 1.5 mm to 5 mm and more especially in the range 2 mm to 3 mm.
In the float process for flat glass manufacture, molten glass is delivered at a controlled rate on to one end, the hot end, of a molten metal bath contained in an elongated tank structure. Usually the molten metal bath is of molten tin or of a molten tin alloy in which tin predominates.
The final ribbon of glass is discharged from the bath by traction means, usually driven traction rollers, disposed beyond the outlet end of the bath, which traction means applied tractive force to advance the ribbon along the bath.
2. Description of the Prior Art
In some ways of operating the float process, regulation of the applied tractive effort is effected along with regulation of the thermal conditions to which the advancing ribbon of glass is subjected so as to attenuate the ribbon to a desired width and thickness. Outwardly and longitudinally directed marginal forces may be applied to the glass while it is being attenuated so as to control gradual and progressive reduction of width and thickness of the ribbon until the ribbon of glass reaches a desired width and thickness. The thermal control is such that when the desired width and thickness of the ribbon is achieved, the viscosity of the glass is at a value at which further dimensional change cannot take place under the applied tractive effort.
Usually the thermal regime to which the glass is subjected is such that the ribbon of glass passes along a zone of the bath where the viscosity of the glass is controlled to regulate the attenuation of the ribbon and in this zone of the bath the glass accelerates as the controlled attenuation takes place.
When operating under high load conditions for example at a rate of delivery of molten glass to the bath of 2000 tonnes per week or more, the speed of discharge of the ultimate ribbon of glass from the bath is high, for example 10 meters per minute up to 40 meters per minute. Such high speeds are necessary when attenuating the glass under these high load conditions to thicknesses below 3 millimeters, for example in the range 2 mm to 3 mm.
The advancing ribbon of glass, when it is accelerating during attenuation and when it is advancing at uniform high speed for discharge from the bath, entrains an appreciable quantity of the molten metal of the bath along the bath surface towards the outlet end of the bath which surface flow induces an upstream return flow of cooler molten metal from the outlet end of the bath along the bottom of the bath towards the zone of the bath where the ribbon of glass is being attenuated and is at a viscosity such that it is particularly susceptible to temperature variations across the surface of the molten metal bath. It has been found that distortion introduced into the underface of the ribbon of glass in this attenuation zone is present in the ultimate ribbon.
One way of minimising distortion in the underface of the ribbon when producing thin float glass at high speeds is to create mixing currents in the molten metal of the bath in the attenuation zone so as to combat localised temperature gradients along the glass-supporting molten metal surface and thereby minimise surface distortion in the attenuated ribbon. These mixing currents have been created by electromagnetically inducing transverse surface flows of molten metal across the bath surface in that zone.
It has now been found that when producing float glass at loads above 2000 tonnes per week the undersurface of the glass is particularly subject to distortion known as "bands" which takes the form of corrugations in the glass which may be associated with some thickness variation. These so-called "bands" may be a few centimeters wide and a meter or more in length.
It has also been found that a ribbon of thin float glass travelling at a relatively high speed along the outlet end of the bath may be subject to instability in position on the bath by movement of the ribbon from side to side in a periodic manner. This phenomenon known as "snaking" may build up to an amplitude of up to 30 cm to either side of the centre line of the bath with a periodicity of about 5 or 6 minutes. Such movement of the ribbon is undesirable, particularly when the process is operating with automated cutting at the outlet end of the lehr.
It is a main object of the present invention to provide an improved method and apparatus for manufacturing flat glass on a molten metal bath which minimises these difficulties associated with the high speed production of thin float glass.